1. Field of the Invention
The present invention relates to a high-density recording medium, and a method of access thereto. More particularly, the present invention relates to a method of access to a recording-reproducing region in recording and reproduction of data by employing scanning tunnel microscopy (STM) for access and tracking with a probe.
The present invention further relates to an information-processing apparatus and an information-processing method which enable high-density recording and high-speed reproduction.
2. Related Background Art
In recent years, memory materials are utilized in a variety of application fields such as computers and related apparatuses, video discs, digital audio discs, and so forth, and are the key materials in electronics industries. Therefore, the development of the memory materials is actively being made. Generally, memory materials are required to have the performances below depending on the uses:
(1) large recording capacity in small volume,
(2) quick response in recording and reproduction,
(3) low power consumption,
(4) high productivity and low cost, and so forth.
Heretofore, magnetic memory devices and semiconductor memory devices which employ a magnetic body or a semiconductor as the base material have been principally used for the memory systems. As the results of the recent progress of laser technique, inexpensive and high-density recording mediums have come to be used which utilizes optical memory employing an organic thin film of an organic pigment, a photopolymer, or the like.
On the other hand, scanning tunnel microscopy (hereinafter referred to as "STM") has been developed which enables direct observation of the electronic structure of an atom on a surface of a conductor (G. Binnig et al., Phys. Rev. Lett. 49, 57 (1982)). The STM has enabled measurement of a real spatial image of an amorphous substance as well as a single crystal with remarkably high resolution. Further the STM has enabled observation of a variety of samples without damaging it by electric current and with low electric power in atmospheric environment. Therefor the STM is promising in a wide application fields. The STM utilizes the tunnel current which flows through a metallic probe (or a probe electrode) when the metallic probe is brought close to an electroconductive substance at a distance of about 1 nm. This current is extremely sensitive to the change of the above distance, so that various information on the entire electron cloud in a real space can be read by scanning with a probe so as to maintain the tunnel current constant. The resolution in the plane direction is approximately 0.1 nm.
Accordingly, application of the principle of the STM will make it possible to conduct high-density recording-reproduction in a dimensional order of an atom (sub-nanometer). For example, EP A0174860 discloses a recording-reproducing apparatus which allows writing by eliminating atom particles adsorbed on a surface of a medium by employing an electron beam or the like and allows reproduction of the data by employing STM. U.S. Pat. No. 4,575,822 discloses a method of recording by injecting electric charges into a dielectric layer formed on the surface of the medium by employing tunnel current flowing between the surface of the recording medium and a probe electrode, and a method of recording by physical or magnetic destruction of the surface of the recording medium by employing a laser beam, electron beam, a corpuscular beam, or the like.
EP A0272935 discloses a recording-reproducing method in which recording and reproducing of information is conducted by employing STM on a material giving memory effect in switching characteristics of voltage or electric current as the recording medium, for example, a thin film layer of a .pi.-electron type organic compound or a chalcogenoid compound. This method enables a large capacity of recording and reproduction of as dense as 10.sup.12 bits/cm.sup.2.
In practical apparatuses, in order to record or reproduce data to or from a memory medium, a process of "tracking", namely positioning and tracing of data lines, is necessary. Tracking methods for high density memory include:
(1) a method of writing preliminarily a marker at the reference position [Japanese Patent Application Laid-Open Nos. 64-53363, and 64-53365]
(2) a method of forming preliminarily a V-shaped groove on the surface of a recording medium, and controlling a probe electrode so as to trace continuously the center of the groove: in recording, the groove being formed and simultaneously recess-projection information being recorded in the groove, and in reading, the groove being traced [Japanese Patent Application Laid-Open Nos. 1-107341 and 151035]
(3) a method of writing or reading information along a crystal lattice arrangement in a stripe shape by utilizing arrangement of atoms in a crystal [Japanese Patent Application Laid-Open No. 1-154332].
(4) a method comprising embedding an electroconductive tracking electrode into a recording medium and conducting tracking by utilizing a tunnel current flowing between the electrode and a probe electrode [Japanese Patent Application Laid-Open No. 1-133239].
However, the aforementioned methods (1) to (4) involve disadvntages respectively as below.
In the method (1) in which the reference position is preliminarily written in, the recording data can be recorded two-dimensionally or areally and the recording density is exceedingly high owing to the high resolution characteristics of STM. In this method, however, the whole surface of the recording medium have to be scanned two-dimensionally to detect the reference position marker, which takes considerable time before the marker is detected.
In the method (2) in which a groove is formed for tracking on the surface of the medium, the groove formed is required to be in the size of 30 nm to meet the recording pit size of around 10 nm. One way to realize it is working with an ion beam. For the working of fine grooves of as small as 30 nm in a pitch in a range of from 30 to 60 nm, a super-precise stage has to be employed which works with mechanical accuracy of 3 to 6 nm. Such an apparatus is of an exceedingly large size and requires restricted operating environment, being unsuitable for memory mediums which are to be mass-produced at low cost. Although the groove may be formed by working with an electron beam or by X-ray exposure, the width of the groove formed cannot be made smaller than approximately 0.3 .mu.m, which makes impossible the high-density recording by utilizing the high resolution of STM.
In the method (3) in which the tracking is conducted against the atomic arrangement in crystal, the high resolution of STM can be well utilized. However, a crystalline substrate cannot readily be obtained which has a complete lattice arrangement in stripe without disorder or defect throughout the required recording area of such as 1 cm.sup.2 corresponding to a recording density of 10 nm/bit and a recording capacity of 10.sup.12 bits. Therefore this method cannot satisfy the requirements of low cost and high productivity for the recording medium.
In the method (4) in which a tracking signal is applied to an embedded tracking electrode and the tracking is conducted by detecting the signal with a probe, the control mechanism may be simple for a recording-reproducing apparatus. However, the provision of such an electrode and a terminal for injecting tracking signals of two or three kinds in the medium results in extremely low recording density and makes the preparation process of recording mediums complicated, which lowers the productivity and the yield of the medium, preventing supply of the mediums at low cost.